Method of forming an oxide coating on metals



March 12, 1968 R. L- EVERY Filed March 2, 1966 WEI ACTIVE FIGURE I /THERMOMETER RANS-PASSIVE 4 Sheets-Sheet 1 TEST ELECTRODE CALOMEL ELECTRODE STOPPER POTASSIUM CHLORIDE SOLUTION 54 T FIGURE 3 PASSIVE DESIRED SAFE OPERATING RANGE RANGE INVENTOR.

FIGURE 2 RICHARD L EVERY ATTORNEY Mmh 12, 1968 Filed March 2, 1966 MILLIVOLTS MILLIVOLTS NOBLE NOBLE NOBLE R. L. EVERY 3,373,093

METHOD OF FORMING AN OXIDE COATING ON METALS 4 Sheets-Sheet 2 PI-PI0 IN 67% H SO +Fe(54 ppm. +500 C so so 70 so 90 I00 II0 I20 FIGURE 4 PIPI0 IN PICKLE LIQUOR +390 20 0 so 40 e0 70 s0 90 I00 II0 I20 FIGURE 5 +lOOO GOLD- GOLD OXIDE +000 x PLATINUM PLATINUM OXIDE CIRHODIUM- RHODIUM oxIoE TEMPERATURE APPROXIMATELY 25C 40 so a0 I00 II0 INVENTOR RICHARD L. EVERY PER cENT SULFURIC ACID BY ATTORNEY March 12, 1968 Filed March 2, 1966 MILLIVOLTS MILLIVOLTS R. 1.. EVERY 3,373,093

METHOD OF FORMING AN OXIDE COATING ON METALS 4 Sheets-Sheet 5 Lu +540 ..1 CD 2 +420 6 Pi-PIO IN 85% H3PO4NO Fe C 4o so s I00 no |30c FIGURE 7 w so .J a: O z -|30 Pt-PTO IN 20% N0 OH NO Fe 20C 30 40 so so 7o 80 9o 7 I00 no I20 C FIGURE 8 Lu I II! 2 +470 Pr-mo IN 67% H2504 NO Fe AT |o0c x Pt-PfO IN 67% H2804 NO Fe AT 25C +430 0 I0 20 30 40 so so 70 so 90 I00 340 HOURS FIGURE 9 +4 m 5O Pt-PiO IN 85% H3PO4 NO Fe AT 25C g 8 2 2 +410 1 5 +370 0 1o 20 30 40 so so 70 so so we HOURS FIGURE IO INVENTOR.

RICHARD L. EVERY Jim AT'ITORNEY R. L- EVERY March 12, 1968 4 Sheets-Sheet 4 Filed March 2, 1966 IOO0.0

1020 MILD STEEL m NuHSO TMOLTEN) AT 420F 2 (DATA POINTS TAKEN AT 2 MIN. INTERVALS) ODIC CURREN |o.o 100.0 CURRENT DENSlTY(MA/IN.

FIGURE I! Am 0 F E V R U C N w T A .1 R A L O D. m D O N A OEI O O O 0 w 0 O O O w w 8 4 4 8 0 6 2 I m .-0 mJmOZ 304 STAINLESS STEEL IN No (MOLTEN) AT 420F cuomi wQxO Elm Oh 2302 3;. 2. JSPZmhOm E'ATFTGBT' FU'R'RENT CURRENT DENSITY (MA/1N2) FIGURE 12 ATTORNEY United States Patent 3,373,093 METHOD OF FORMING AN OXIDE COATING 0N METALS Richard L. Every, Ponca City, Okla, assignor to Continental Oil Company, Ponca City, Okla, a corporation of Delaware Continuation-impart of application Ser. No. 368,956, May 20, 1964. This application Mar. 2, 1966, Ser. No. 531,261

2 Claims. (Cl. 204-39) This application is a continuation-in-part of application for United States Letters Patent Ser. No. 368,956, filed May 20, 1964, now abandoned, which was a continuationin-part of application for United States Letters Patent Ser. No. 260,804, filed Feb. 25, 1963, now abandoned. The invention relates to a method of putting an oxide coating on a metal. This invention also relates to a method of making metal-metal oxide electrodes. The electrodes produced by this method are useful as reference electrodes in the determination of the susceptibility of a metal to attack by corrosive electrolytic solutions. Such methods of determining the susceptibility of a metal to attack by corrosive electrolyte solution form the subject matter of my co-pending application No. 368,956 mentioned above.

In the anodic polarization systems known to me to be in commercial use at the present time, two types of electrochemical halfcells are generally employed as the reference electrode. Of these the most frequently used is the calomel half-cell, an electrode accepted as a standard and widely used in other applications by virtue of its relatively stable and constant potential. The other reference electrode which is used in the anodic polarization corrosion system is a silver-silver chloride electrode. The calomel electrode is a liquid electrode while the silver-silver chloride electrode is solid. I

Although corrosion control systems employing these two types of reference electrodes to determine corrosion susceptibility have generally worked satisfactorily under certain preselected conditions, these electrodes lack universality with respect to the types of electrolytes in which they may be used; and each suffers from serious limitations when subjected to variations in the concentration of the electrolyte in which it is used or to varying temperature conditions.

The silver-silver chloride half-cell is rather difficult to prepare properly and because of its softness, presents some difficulty in mounting. The most serious shortcomings of this reference electrode, however, are its solubility in oleum, a frequently encountered electrolyte in such corrosion protection systems, and its susceptibility to severe erosion in highly agitated systems.

Although the calomel electrode is of more universal utility than the silver-silver chloride electrode, in several respects it presents serious disadvantages not shared by the silver-silver chloride electrode. For the most part, these stem from the fact that it is a liquid electrode; thus to prevent contamination of the electrode, a suitable electrochemical salt bridge must be utilized to provide a conductive path between the corrosive electrolyte and the calomel electrode. Not only are reference electrode assemblies employing such salt bridges difficult to install but they also present a problem of contamination of the electrolyte product by the salt solution of the bridge; more over, where large tanks filled with a corrosive electrolytic chemical are to be protected, the salt bridge must be of considerable length; and for this reason, it is rather fragile and is, therefore, subject to mechanical malfunction particularly in systems in which the corrosive electrolyte is violently agitated. Also, irl product storage systems where the corrosive electrolyte is maintained under pressure, some means must be provided for equalizing the pressure acting upon the salt solution and the electrolytic bridge.

3,373,093 Patented Mar. 12, 1968 The ideal reference electrode for use in determining susceptibility of corrosive attack must possess certain characteristics, The properties of the ideal reference electrode for use in anodic polarization corrosion control systems may be summarized as follows:

(a) Insolubility in any electrolyte in which it may be used and in any concentration of such electrolyte;

(b) That it exhibit an electrical potential which is essentially independent of electrolyte concentration and temperature;

(c) That it does not require use of an electrolytic salt bridge; and

(d) That it can be fabricated in the form of a durable structure which will resist erosion, abrasion and impact.

In addition to the use as electrodes of metal-metal oxides prepared by this method, other uses have been found for the oxide coated metals. Exemplary of other uses of the oxidized metals are: use in batteries and fuel cells, as elements in electron emission, as electrolytic rectifiers, capacitors, use in catalysis, in electrochemistry and as a protection of the metal from corrosion and erosion.

An object of the present invention is to provide an improved electrode and methods of making the same which will have properties closely approaching the above-mentioned properties of the ideal reference electrode.

Another object of the invention is to provide new reference electrodes for use in determining the susceptibility of metals to corrosive attack, which electrodes may be installed and used in anodic polarization corrosion control systems by being placed directly in contact with the corrosive electrolyte and without requiring the use of a salt bridge.

A further object of the invention is to provide a method for making certain electrodes which are particularly useful in determining the susceptibility of metals to corrosive attack when in contact with corrosive electrolytes.

An additional object of this invention is to provide a method of coating a metal with an oxide which is several molecular layers thick and tightly adherent to the base metal. 1

Other objects and advantages of the invention will become apparent upon reading the description of the invention which follows.

According to the present invention, there is provided a method for the preparation of oxides on metals, said method comprising the step of immersing the metal in a molten oxidizing salt, such as an alkali metal nitrate or alkali metal chlorate, making the metal the anode in an electrolytic circuit including the alkali metal salt, and passing sufficient current through the eletcrolytic circuit to form an oxide coating on the metal.

The characteristics of the individual electrodes in various environments and the nature of the anodic system of corrosion prevention are disclosed herein with the aid of illustrative drawings and graphs as outlined below:

FIGURE 1 is a schematic illustration of a typical anodic polarization corrosion control system.

FIGURE 2 is a graph illustrating a typical polarization curve for a metallic vessel to be protected from corrosion by contact with a corrosive electrolyte contained therein.

FIGURE 3 is a schematic diagram illustrating the apparatus which I have utilized in observing the stability of the potential of the reference electrodes of the present invention by measuring the potential difierence between the electrode under test and a calomel reference halfcell maintained at approximately 25 C.

FIGURE 4 is a graph in which potential is plotted against temperature for a platinum-platinum oxide electrode in contact with 67% sulfuric acid.

FIGURE 5 is agraph in which potential is plotted 3 against temperature for a platinum-platinum oxide electrode in direct contact with pickle liquor electrolyte.

FIGURE 6 is a graph in which potential is plotted against acid concentration for certain noble metal oxide electrodes when in contact with a sulfuric acid electrolyte V maintained at 25 C.

FIGURE 7 is a graph in which potential is plotted against temperature for a platinum-platinum oxide electrode in direct contact with 85% phosphoric acid.

' FIGURE 8 is a graph in which potential is plotted against temperature for a platinum-platinum oxide electrode in contact with 20% sodium hydroxide.

FIGURE 9 is-a graph in which potential is'plotted against time for platinum-platinum oxide electrodes immersed in sulfuric acid solutions maintained at difi'erent temperatures.

' FIGURE 10 is. a graph in which potential is plotted against time in order to illustrate the stability of a platinum-platinum oxide electrode in the 85% phosphoric acid over extended periods of time.

FIGURE 11 is a graph showing a polarization curve for 1020 mild steel in contact with molten sodium hydrogen sulfate and using a platinum-platinum oxide reference electrode.

FIGURE 12 is a graph showing a polarization curve for 304 stainless steel in contact with molten sodium hydrogen sulfate and using a platinum-platinum oxide reference electrode.

Referring now to the drawings in detail and particularly to FIGURE 1, a typical anodic polarization. corrosion control system generally comprises a metallic vessel 10 or other metallic member which it is desired to protect from the corrosive influence of a corrosive electrolyte 12 which is in contact therewith. The anodic polarization system per se comprises an inert electrode 14 which is suspended in the corrosive electrolyte 12 and Which is made a cathode with respect to the metallic vessel 19 which is connected as the anode in a suitable electrical circuit. The electrical circuit which includes the cathode 14 and the vessel 10 comprises a suitable source of direct current 16, electrical leads 18 and 20 connected to the cathode 14 and the vessel 10 respectively, and a suitable switch 22 for opening and closing the electrical circuit.

The control of the closure of switch 22 and consequently the passage of current between the vessel 10 and the inert cathode 14 is efiected by means of a reference electrode 24- and suitable control circuitry 26. The reference electrodes which have previously been used have generally required the use of a suitable electrolytic salt bridge, which is designated in FIGURE 1 by reference character 28, in order to place the reference electrodes in electrochemical communication with the corrosive electrolyte in the vessel 10.

As a potential of the metallic vessel 10 is varied with respect to the reference electrode 24 the susceptibility of the vessel to the corrosive attack by the corrosive electrolyte 12 is also varied. An indication of the passivity of the vessel or its immunity to corrosive attack can therefore be determined by observing the variation in the potential' difier'ence between the reference electrode 24 and the metallic vessel 10. Since the potential of a properly ft'inctioning reference electrode remains essentially constant, variations in the potential difference between this electrode and the vessel '10 will be indicative of a change in the potential of the vessel 10 and hence a change in its's'usceptibility to corrosive attack. The controller 26 converts the variation in this potential difference to control signals which operate the switch 22 so that current is passed from the electrolyte between the vessel 10 and the inert cathode 1-4 at such times as may be required to maintain or restore the metal of the vessel 10 to a passive state.

- In FIGURE 2 of the drawings, a typical anodic polarization curve is illustrated. This curve will be characteristic Qt ea p r icular vessel and electrol te system which is under the protection of the system. It will be noted that over the range of potentials at which the vessel is passive very little current is required to be passed between the vessel and the inert cathode in order to maintain the vessel at these passive potentials. 'On the other hand, to pass from an active potential to the range of passivity a large amount of current must be applied to cause the potential to change through the fiade point. It should also be noted that most electrolytic systems requiring protection will, as shown in FIGURE 2, have a relatively large range of potential over which the vessel will be passively or relatively inert (noble) with respect to the electrolyte. For this reason, an isothermal potential deviation of :50 millivolts or more can frequently be tolerated in the reference electrode without the occurrence of detrimental errors in the determination of the time and quantity of current which should be passed.

As previously indicated, it is one of the major objects of the present invention to effect improvement in anodic polarization corrosion control systems of the type illustrated in FIGURE 1 by providing reference electrodes which function in a manner superior to the calomel and silver-silver chloride electrodes which have previously been utilized for this purpose. In order to evaluate the extent to which a number of materials might approach ideality or at least suitability in their ability to function as reference electrodes, a great many materials have been tested by me in a variety of corrosive electrolytes which generally include those most frequently encountered in chemical storage systems of the type where corrosion protection is most essential. The electrolytes which were employed in evaluating possible reference electrode materials were: sulfuric acid, oleum, phosphoric acid, poly-phosphoric acid, pickle liquor, sodium hydroxide, chromic acid and molten sodium hydrogen sulfate. Varying concentrations and temperatures of each of these electrolytes were utilized; and in the case of most of the electrode materials tested, quantities of iron were dissolved in the electrolytes to determine the effect, if any, this material might have upon the constancy of the potential of the material tested as electrodes. The provision of substantial quantities of iron in solution in the electrolytes, in many instances, was efiected for the purpose of simulating more closelythe conditions actually obtaining in situations where anodic polarization systems are utilized to preventthe corrosion of vessels constructed of ferrous metals.

In order to accurately evaluate the variations in potential of the materials which were tested for suitability as reference electrodes, the testing apparatus illustrated schematically in FIGURE 3 was devised. My previous work prior to design of the apparatus illustrated in FIG- URE 3 had shown that the deterioration of agar salt bridges interposed between a corrosive electrolyte and a reference calomel electrode for the protection of the calomel electrode had caused the measured between the material under test and the calomel electrode to drift. The apparatus illustrated in FIGURE 3 virtually eliminates mixing of the salt bridge solution and the corrosive electrolyte being used in the test of the electrode materiaL- Referring to FIGURE 3, a vessel 30 constructed of glass or other suitably inert material is used to contain a corrosive electrolyte 31 utilized in the test. This vessel is provided with a stopper 32 which carries a thermometer 34 and a test electrode material 36, both of which extend into contact with the corrosive electrolyte solution 31 in the vessel 30. The vessel 30' is placed on a thermostatically controlled heater 38 which facilitates the control variations of the temperature of the corrosive electrolyte. A .calomel electrode 40 is placed in communication with a saturated potassium chloride solution 44 contained in a suitable vessel 46. Electrically conductive communication is established between the corrosive elec trolyte 31 and a saturated potassium chloride solution 44 in the vessel 46 by means of side arms 43 and 50 extending from the vessels 30 and 46 respectively and communicating with additional saturated potassium chloride solution 52 contained within a third vessel 54. Conincluding the molten salt bath and electric current was passed through the circuit to form an oxide coating on the metal. In the electrodes prepared by this method, the amorphous coating was lusterless and without apparent trol of the flow of the corrosive electrolyte in a side 5 color. Analytical evidence indica e he presen e in he arm 48 is achieved by means of stopcocks 56 and 5S coating of mixed platinum oxides. and by a vent arm 60 communicating with the side arm Spectrophotometric examination of the oxide films of 48 and containing a stopcock 62. A stopcock 64 is the platinum-platinum oxide electrode shows that the interposed in the side arm 50 to control the flow of satuaverage m COIlSiStS of a mlXhlfe 0f Plo and a in the rated potassium chloride solution between the vessel 46 1Q ratio range of about 3:1 to about 7:1 respectively. The and the vessel 54. All of the stopcocks are maintained in fi m t ickness has been found to vary from about 12 to a grease-free condition. The calomel reference half-cell 157 at mi layers. 40 is maintained at a temperature of approximately In FIGURE 4 the results of potential stability tests Of 25 C. and the potential difference between the test eleca P -p ati OXide l tr de in 7% Sulfuric acid trode and the calomel reference half-cell may be meascontaining 54 Parts P milliOn n a n p rtrayed ured with a Millivae D,C. Voltmeter (T e MV 17C), in the form of a graph. The temperature stability of this a, Keithley Eleetromete (Type 610A) th it bl electrode is seen to be comparable to that of the calomel measuring device. electrode. The range of measured potential over a tem- A number of rever ible ele trode cQmPfi ing a perature range of 100 C. is from about 390 millivolts to tallic base with a metal oxide coating thereon were te t d 20 490 millivolts. Despite this slight temperature instability for suitability as reference electrodes in anodic polariof the platlhllmrplatlhhm OXldfi electrode in a Sulfuric zation corrosion ontrol systems, acid electrolyte containing iron, the many other advan- These electrodes were prepared b immersing d f tageous qualities of this electrode make it one of the platinum, hodi ta t l palladium, h f i ld, most suitable for use in anodic polarization corrosion molybdenum, tungsten, chromium, zirconium and niobium Control Systems; thus, it Will be perceived y reference individually i a lt lt b th, ki h metal to FIGURE 5 that the platinum-platinum oxide electrode the anode in an electrolytic circuit including the alkali Performs l in a Shfhflc Pickle liquor eleclfolyte- Its metal salt and passing sufiicient current through the elecmean Potenllal h y ls also excellent in 20% Sodium trolytic circuit to form an oxide coating on the metal. y hh as lhdlcated in FIGURE The temperature Th id ti were id tifi d i h b X ray or stabihty of the platinum-platinum oxide electrode in 85% spectrophotometric procedures. The exact temperature p p acid y 111 which iron s isof the molten salt was not critical in the formation of SOlVeCl, Whllfi satlstactory, 15 not as good as the Stability th o id th l Prerequisite; b i h h salt be of this electrode in other electrolytes in which it was in the molten state. The period of time required to form tested- The results of potehhhl meashrfilhehts made While th id i d between about 1 and about 5 minutes the electrode was in direct contact with the phosphoric the exact time being dependent upon the particular metal acld electrolyte l portrayed 111 the graph illustrated in being oxidized and the amount of current density applied FIGURE It Wlll he holed that h temperature range to the metal, While current densities varying between over which the test was conducted 1n obtaining the FIG- about 1 and about 50 ma./ sq. cm. represent the practical URE 7 data Was from about tllfollgh C- operating range, the exact current density required in In ohder to elfalhate h long-term electrode Stability any given case depends upon the time Such current is of platinum-platinum oxide electrodes, tests were conapplied. When low-current densities are utilized, the time ducted l eYaluate lhls pal'hmeter "when the electrode Was i d to f h desired oxide will be longer than placed n direct contact wlth 67% sulfuric acid containwhen high-current densities are utilized. The rate at which f l malhtalhed h 9 57% Sulfilficiacid the oxide coating is formed also determines the type cohtalhlhg I10 and mhlhtalhed at and 5% of oxide which will be present in any given situation. Phosphoflc hcld Cohthlhlhg 110 iron and maintained all Where the coating is applied rapidly, the resulting coat- 5 C. The results of these tests are plotted in the graph ing is a rather loose and porous structure. Likewise, Shown in FIGURES 9 and 10. It will be noted that in where the coating is formed slowly, the resulting coateach case the stability of the potential of the electrode ing is dense and tightly adherent. The results are prein these electrolytes over extended periods of time was sented in Table A. excellent, being in every instance as good or better than TABLE A C trout in Example Metal Molten Salt Temp, C. Dgnsity, Time, min. Appearance Coating llie fh iiii i l d in maJsq. cm. Formed Analyzing Product 1 Platinum KNO; 400 1 1 0 v i 400 10 1 U.V. 400 1 1 U.v. 400 10 1 U.V. 400 1 1 X- v 400 10 1 Do. 400 1 1 D0. 400 1 1 Do. 400 10 1 Do. 400 1 1 Do. 420 2 5 Do. 400 s 1 Do. 400 2 1 Do. 400 so 1 Do. 400 20 1 Do. 400 20 1 Do. 400 5 5 do Do. 400 31 5 Silvery-gray do Do.

1 Beckman DU Spectrophotometer at 230 and 260 mv.

1 Platinum having a previously applied coating of platinum black deposited thereon.

the long-term electrode stability of the calomel electrode. In addition to the test of the platinum-platinum oxide electrode which yielded the results plotted in the graph shown in FIGURES 4, 5, 7 and 10, this electrode was platinum was made the anode in an electrolytic circuit also tested to determine the stability of its potential in phosphoric acid solutions in varying concentrations. The results obtained from measurements of the E.M;F. relationship between the platinum-platinum oxide electrode and the reference saturated calomel electrode at approxi- .mately 25 C. is tabulated in Table B. It will be seen from the data in this table that the platinum-platinum oxide electrode is relatively insensitive to changes in phosphoric acid concentration and even maintains its potential stability in polyphosphoric acid containing free phospho- A test was also conducted to determine the stability of the potential of a platinum-platinum oxide electrode in a chromic acid cleaning mixture of the type commonly employed in cleaning laboratory glassware. This cleaning mixture consisted of 92 grams of sodium dichromate dihydrate, 458 grams of distilled water, and 800 ml. of concentrated sulfuric acid (96% The time required for the electrode to become stabilized in the chromic acid cleaning mixture was evaluated and the results of this evaluation are tabulated in Table C below.

C.PLATINUM-PLATINUI\I OXIDE ELECTRODE IN CHROMIC ACID CLEANING MIXTURE EMT. referenced to calomel, in millivolts TABLE Time, minutes:

It will be perceived that the platinum-platinum oxide electrode came readily to a stable value and retained this over an extended period of time. This result confirms the extended time stability of the platinum-platinum oxide electrode observed in the sulfuric acid electrolyte and represented by the graph depicted in FIGURE 9.

In addition to the platinum-platinum oxide electrode, a rhodium-rhodium oxide electrode was prepared in the manner similar to the preparation of the platinum-platinum oxide electrode. A strip of rhodium metal was dipped in a bath of fused potassium nitrate maintained at a temperature of about 400 C. The rhodium was madethe anode in an electrolytic circuit including the molten salt bath and electric current was passed through the circuit to form an oxide coating on the metal. The type of oxide whch was formed by this procedure was determined by X-ray defraction to be Rh O Upon testing the rhodium-rhodium oxide electrode in sulfuric acid electrolytes of varying temperature and concentration, a behavior substantially identical to that exhibited by the platinum-platinum oxide electrode resulted. Temperature stability of the electrode was very good; however, in the case of both the rhodiurnrhodium oxide and platinum-platinum oxide electrodes, the potential of the electrodes remained substantially constant over a range of lower concentrations of the sulfuric acid electrolyte then changed sharply at a concentration of approximately 96% of the acid and then once again assumed stability in concentrations of from approximately 100% to 115% sulfuric acid. These tests of electrode potential stability against sulfuric acid concentration indicate that the rhodium-rhodium oxide and platinum-plati num oxide electrodes can be used to excellent advantage in a corrosive sulfuric acid electrolyte of less than 96% acid concentration or of greater than 100% acid concentration (oleum). Between these two ranges of acid concentration, however, a factor must be introduced to compensate for the change in electrode potential with changes in acid concentration. A preferred use of these two noble metal oxide electrodes would therefore appear to be in oleum service and sulfuric acid service where the concentration of the sulfuric acid is less than 96%.

In addition tot he electrodes made from noble metals rhodium and platinum, a gold-gold oxide electrode was also fabricated in a similar manner and appeared tov possess electrode characteristics quite similar to that of the rhodium-rhodium oxide and platinum-platinum oxide electrodes. Thus, in FIGURE 6, it may be observed that the potential of the gold-gold oxide electrode also is quite stable for sulfuric acid or oleum concentrations of less than 96% and greater than 100% respectively. The stability of the gold-gold oxide in oleum is evidenced by the data tabulated in Table D.

It is apparent that the electrode maintains a substantially constant potential as the percentage of free sulfur trioxide in the oleum electrolyte is varied between 15% and 33 This stability of the noble metal oxide electrode in oleum solutions is especially desirable in view of the widespread use of this electrolyte in various chemical manufacturing and processing industries.

One of the most desirable characteristics of the goldgold oxide electrode is its ability to become readily stabilized in most of the electrolytes in which it was tested. In this respect the gold-gold oxide electrode was superior to either the platinrim-platinum oxide electrode or the rhodium-rhodium oxide electrode. Typical data showing the effect of aging on the potential of the gold-gold oxide electrode are tabulated in Table E which shows the effect of maintaining the electrode in contact with an oleum electrolyte containing from 30% to 33% free sulfur trioxide over a period of 24 hours. i

TABLE E i h EMF. referenced to calomel, in millivolts The data taubulated in Table E indicate that within 7 15 minutes of its being placed in contact with the oleum electrolyte the gold-gold oxide electrode has attained'the TAB LE F Acid concentration percent H PO E.M.F. referenced to calomel, in millivolts The results which are set forth in Table F represent only approximate values obtained for the goldgold oxide electrode because of the small size of the electrode used; however, the data are considered suf- 9 ficiently accurate to show that the electrode potential is essentially independent of the concentration of the phosphoric acid electrolyte.

Like the platinum-platinum oxide electrode the goldgold oxide electrode was also tested for potential stability milli-inches per year or 93.3 These results clearly demonstrate the eiiectiveness of the noble metal-noble metal oxide electrodes as reference electrodes for use in anodic polarization corrosion control systems for protecting steel vessels against corrosion by molten salts.

over extended periods of time when placed in contact with Another type of metal-metal oxide electrode was proved, a chromic acid cleaning mixture. The composition of the by the test which I have conducted, to possess properties mixture was the same as that described above in connecindicating its suitability for use as -a reference electrode in tion with the description of the platinum-platinum oxide the anodic polarization corrosion control systems. This tests. The data obtained in conducting this test are tabuwas a molybdenum-molybdenum oxide electrode. In a latedin Table G. test of this material the relationship between the TABLE G E M F a d t molybdenum-molybdenum oxide electrode and a saturated T1me,mlnutes: fi fififf g calomel reference electrode maintained at 25 C. was

0 1 5 determined while the molybdenum-molybdenum oxide 5 0 electrode was in contact with phosphoric acid solutions of 0 1 70 different concentrations. As shown by Table I below, the 40 1370 of this electrode with respect to calomel is suit- 9 7 ably independent of the strength of the phosphoric acid 5 7 solution as the acid concentration is varied from 85% It is again apparent that the gold-gold oxide rapidly 2O to113%' attains, and then continues to maintain, an excellent sta- Acid TABLE I concentratlon blllty over extended penods of time. ercant H PO EM refe e ced to Another extremely desirable property of the noble p 85 9 3 calomel m fgg metal oxide electrodes which should be mentioned is their 190 complete immunity to attack by the corrosive electrolytes 0 220 which were utilized. No tendency towards solubility in the electrolytes was observed. The molybdenum-molybdenum oxide electrode was In additio to their u fuln i lf i id 1 also evaluated for its ability to quickly attain equilibrium phoric acid, and ch o i id h bl t l id 1 and to maintain its stability over an extended period of trodes were found to be useful reference electrodes in a time. Typi data Showing the effect of g g 011 the P system used to protect metallic vessels from corrosive tefltifll [he molybdenum-molybdinum OXide electrode are tt k b lt lt solutions d ti l l molten tabulated in Table J. The tests were conducted with the acid salts such as sodium hydrogen sulfate. Since substanelectrode in Contact with a 115% PhBSPhOIiC acid $0111- tially all of the water is normally driven off of the salts in tion maintained at 25 the molten state, no problem of variation of electrode potential with electrolyte concentration is encountered TABLE J D .M.F. referenced to in thls application. Time, hours: calomel, in mimvons Because of the high temperatures encountered in the use 0 190 of anodic polarization systems to protect metals against /2 200 attack by molten salts, it is not possible to evaluate the 2 210 stability of the noble metal-noble metal oxide electrodes 3 220 by comparison with the standard calomel electrode. At 4 220 the temperatures at which many salts are molten, the salt 5 /2 220 bridge electrolyte solution would be caused to boil. How- 22 /2 220 ever, polariaztion curve and corrosion test data obtained using the platinum-platinum oxide electrode as the refer- AS indicated y the data Set forth in Table the ence electrode clearly indicate the suitability of the noble molybdenum-molybdenum oxide electrode attains q metal-noble metal oxide electrodes for this use. Thus, in libfium p y and maintains a relatively Constant P FIGURES 11 d 12, h l i i curves f 1020 5 tial over extended periods of time. In the test of extended mild steel and for 304 stainless steel in Contact with molten stability, which Was conducted, it was also observed t sodium hydrogen sulfate and using a platinum-platinum this electrode is not appreciably attacked by the phosoxide reference electrode each indicate the existance of PhOIiC 'aCid Solutions ranging in Concentration from 36% a passive potential region of minimum current density. to 115%.

Table H shows the corrosion rates of both unprotected A c o iml m-c m Oxide e ect de Was also prc and anodically polarized 1020 mild steel and 304 stainpared in the manner previously disclosed. This electrode less steel coupons in molten sodium hydrogen sulfate at is very useful in caustic solutions. Table K shows the sta- 420 F. The platinum-platinum oxide electrode was used bility of the chromium-chromium oxide electrode in 50% as the reference electrode in an anodic polarization corsodium hydroxide solution when the solution was heated rosion control system of the type herein before described and illustrated in FIGURE 1. I

TABLE H Liquid Phase 1 Corrosion Rate, m.p.y. Polarization 2 Current 3 Percent Type Metal Potential, mv. Denslty, Protection Unpro- Anodieally Ina/sq. it.

tected Polarized 1020 mild steel 924 4. 0 +600 (noble) 5.8 99. 6 Do 2.3 +750 (noble) 4.4 99 8 304 stainless steel 81 5. 4 +750 (noble) 9. 6 93. 3

1 Approximately 20-hour tests.

2 Pt-PtO reference electrode (420 F.). 3 For maintaining polarization near close of test.

In referring to Table H it will be noted that anodic protection reduced the liquid phase corrosion rate of mild steel from 924 to 2.3 milli-inches per year or 99.8%, and

to 100 C. It should also be noted that the chromiumchromium oxide was not visibly attacked by this concentrated sodium hydroxide solution, even at the elevated reduced the corrosion rate of stainless steel from 81 to 5.4 temperatures.

TABLE K..E'FFECT OF TEMPERATURE ON E.M.F. OF OHROMIUM-OHROBHUM OXIDE Temperature, C. E.M.F., my. Time, Hours 23 +545 11 23 +520 23 +525 31 +530 23% i-MO 23% +550 23% +570 23% +600 23% +635 23% 1 To saturated ealomel at approximately 25 C.

1 Active.

A tungsten-tungsten oxide electrode was prepared in the manner as previously disclosed. This electrode was then tested in a 50% sodium hydroxide solution and the temperature ;was varied over a period of time. The results are shown in Table L.

TABLE L.EFFEGT 0F TEMPERATURE ON E.M.F. OF

TUNGSTEN-TUNGSTEN OXIDE ELECTRODE IN 50 PER- CENT OAUSTIC 2 Active.

These data show: (1) the EMF. was essentially constant at minus 1120 millivolts (nobble) throughout the temperature .range 50 to .135 C.; .(2) the varied slightly at 27 C. (minus 950 to minus 1020 millivolts); and (3) .the shiftedonly to minus ll20irnillivolts as the temperature was increased from 27 C. to 50 C.; thus, the electrode .EMF. is reasonably steble at room temperature, .shifts only slightly as the temperature is increased and is essentially constant over a wide range of elevated temperatures.

A hafniu rn-hafnium oxide electrode was prepared in the manner previously described and this electrode-was tested in a 50% sodium hydroxide solution, over a temperature range of 25 to 80 C., and found to perform in a very satisfactory manner.

Tantalum metal was oxidized according to the method of this invention and the oxide formed was determined by X-ray to be the beta-tantalum oxide. This electrode. was then tested in a 50% sodium hydroxide solution over a temperature range varying from 25 to C. and found to be asuitable reference electrode.

A nickel-nickel oxide electrode, prepared by the method of this invention, gives a reference electrode suitable for use in strong caustic solution.

In addition to the metal oxides previously disclosed, the oxides of manganese, cobalt, and scandium are in cluded within the scope of this invention.

From the foregoing working examples, it will be perceived that I have extensively tested and evaluated a great number of materials to determine their suitability for use as reference electrodes. While these tests and evaluation have been directed primarily to the determination of materials which possess properties which endow them with superior attributes when utilized as reference electrodes in anodic polarization corrosion control systems, it will be apparent that many of the same properties which characterize reference electrode in such corrosion control applications also must characterize any reference electrode when employed in an application or use which requires it to be placed in contact with a corrosion electrolyte. The solid nature of many of the materials which have proved suitable for such use enables them to be usefully employed in applications where substantial mechanical strength and ease of installation are important factors. Moreover, the excellent temperature and concentration stability of most of the electrodes discussed herein before indicates that they possess the properties which are most essential in various analytical techniques where a reference electrode of constant potential is required.

Many of the electrode materials which were tested and which were believed to be sufiiciently good materials for use as reference electrodes appeared to retain their attractive properties in both acidic and basic electrolytes of varying types and concentrations. Others of the electrodes showed definitely better stability in some of the electroentire concentration range of the electrolytes in which they were tested, a very good'stability of these electrodes 7 within certain specific ranges may be advantageously-utilized in a corrosion control situation where the range over which the corrosive electrolyte may vary is known in ad- Vance. It is believed that the preferred application of the several electrodes will be manifest from the foregoing description of the invention and that it is unnecessary to here summarize the properties of the several electrodes which make one electrode or one group of electrodes preferable for use in certain electrolytes or in certain concentrations of electrolytes and other electrodes or groups of electrodes preferable for general and widespread usage where the concentration temperatures and chemical constitution of the electrolytes are not known with certainty in advance'of the installation of the corrosion control system.

In describing the preparation of themetal oxides of this invention by way of examples, certain temperatures, 7

times and current densities were given. It must be understood that the exact temperature ofthe molten salt isnot critical in the formation of the oxides, the only prerequisite being that the salt be in the molten state. The period of time required to form the oxide varied between about 1 and about 5 minutes, the exact time being dependent upon the particular metal being oxidized and the amount of current density applied to the metal. While current densities varying between about 1 and about 50 ma./ sq. cm. and represent the practical operating range, the exact current density required in any given case depends upon the time such current is applied. When low current densities are utilized, the time required to form the desired oxide will be longer than when high current densities are utilized. The rate at which the oxide coating 'is formed also determines the type of oxide which will be present in any given situation. Where the coating is applied rapidly the resulting coating is a rather loose and porous structure. Likewise, where the coating is formed slowly the resulting coating is dense and tightly adherent. Also, in setting forth the practical range of current densities it must be understood that these current densities are those present at the initial current flow. As the 'oxideis formed on the metal, the current is inherently reduced due to the limited conductivity of the oxide coating.

Although a wide variety of applications and usages of the electrode materials which I have for the first time evaluated for use asreference electrodes will occur to electrochemists and others skilled in the art, it is my intention that the novel principles and concepts herein disclosed shall be the criteria establishing the bounds and limits of the present invention. Insofar as minor modifications and innovations are evolved by those skilled in the art for making further use of the novel principles and concepts herein disclosed, it is my intention that such modifications and innovations be considered to fall Within the sphere and scope ofthis invention except insofar 13 as the same may be necessarily limited by the appended claims or reasonable equivalents thereof.

What is claimed is:

1. A method of puttin an oxide coating on metallic nickel comprising:

(a) immersing the metallic nickel in a molten salt consisting essentially of one salt selected from the group consisting of alkali metal nitrates and alkali metal chlorates;

(b) making the metallic nickel the anode in an electrolytic circuit including the alkali metal salt; and

(c) passing sufiicient current through said electrolytic circuit to provide a current density between about 1 to about 50 ma./sq. cm.

2. A method of putting an oxide coating on platinum having a previously applied coating of platinum black deposited thereon comprising:

(a) immersing said platinum in a molten salt consisting essentially of one salt selected from the group consisting of alkali metal nitrates and alkali metal chlorates;

References Cited UNITED STATES PATENTS 2/1958 Yoshida. 3/1966 Hagiwara et al. 20439 X FOREIGN PATENTS 9/1962 Japan.

OTHER REFERENCES Poggendorf Annalen, vol. 72., pp. 481-485, Leipzig, 1847.

HOWARD S. WILLIAMS, Primary Examiner. JOHN H. MACK, Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD OF PUTTING AN OXIDE COATING ON METALLIC NICKEL COMPRISING: (A) IMMERSING THE METALLIC NICKEL IN A MOLTEN SALT CONSISTING ESSENTIALLY OF ONE SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL NITRATES AND ALKALI METAL CHLORATES; (B) MAKING THE METALLIC NICKEL THE ANODE IN AN ELECTROLYTIC CIRCUIT INCLUDING THE ALKALI METAL SALT; AND (C) PASSING SUFFICIENT CURRENT THROUGH SAID ELECTROLYTIC CIRCUIT TO PROVIDE A CURRENT DENSITY BETWEEN ABOUT 1 TO ABOUT 50 MA./SQ.CM.
 2. A METHOD OF PUTTING AN OXIDE COATING ON PLATINUM HAVING THE PREVIOUSLY APPLIED COATING OF PLATINUM BLACK DEPOSITED THEREON COMPRISING: (A) IMMERSING SAID PLATINUM IN A MOLTEN SALT CONSISTING ESSENTIALLY OF ONE SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL NITRATES AND ALKALI METAL CHLORATES; (B) MAKING SAID PLATINUM THE ANODE IN AN ELECTROLYTIC CIRCUIT INCLUDING THE ALKALI METAL SALT; AND (C) PASSING SUFFICIENT CURRENT THROUGH SAID ELECTROLYTIC CIRCUIT TO PROVIDE A CURRENT DENSITY BETWEEN ABOUT 1 TO ABOUT 50 MA/SQ.CM. 